No host defense impairment has been discovered to date in mycosis fungoides patients despite reports of constant cutaneous infections and serious morbidity and mortality from overwhelming bacterial pneumonias and septicemia. Recent development of quantitative assays of monocyte-macrophage function in humans has made it possible to detect defects in this aspect of patients' host defenses. In a pilot study of five mycosis fungoides patients in cooperation with the Mycosis Fungoides Cooperative Study Group we discovered that four patients had defective chemotaxis of their monocyte-macrophages. Further study to confirm and expand upon this observation is needed to determine 1) if detective mononuclear phagocyte function would affect classification and treatment of patients with mycosis fungoides, and 2) if these assays would be useful in determining the effect of chemotherapeutic agents on host defense function and serve as an in vitro method of detecting immunostimulation by various forms of immunotherapy.